Straighten Camera in Blender
The fastest way if you haven't applied any transforms on the camera is to reset the rotation with AltR to 0/0/0 and then rotate it 90&deg; on the X and -90&deg; on the Z, RX 90, RZ -90.

If you have changed the transforms and can't reset it to 0/0/0, you can use exact measurements by using the transform section of the properties panel or the object tab in the properties view to rotate it 90&deg; on the X axis, -90&deg; on the Z axis and 0&deg; on Y.